1. Field of the Invention
The invention relates in general to interferometric profilometry for surface characterization. In particular, it relates to a new approach for rapidly identifying defects on a large sample surface using an adaptive algorithm and single-frame interferometric data.
2. Description of the Related Art
Interferometric profilers have become standard instruments for quality control in the manufacture of many high-precision commercial products. In particular, the ability to inspect large panels (in the order of 60×60 cm2) of plastic substrates rapidly and with nanometer vertical accuracy to identify and characterize defects is critical to the production flow of printed electronics. In particular, for quality-control purposes it is important to distinguish between defects based on their height. Therefore, the identification and characterization of each defect is important. Conventional optical surface profilometry enables the performance of non-contact measurement of small objects with high vertical and lateral resolution and at relatively high measurement speeds. However, the measurement of large objects with current techniques is not fast enough for inline applications.
Widely accepted, conventional techniques are available for calculating surface topography from optical interference data. Phase-shifting interferometry (PSI), for example, is based on changing the phase difference between two coherent interfering beams at a single wavelength, λ, in some known manner, such as by changing their optical path difference (OPD) either continuously or discretely with time. Several measurements of light irradiance with different OPD values, usually equally spaced, at a pixel of a photodetector can be used to determine the phase difference between the interfering beams at the point on a test surface corresponding to that pixel. Based on such measurements at all pixels with coordinates (x,y), a phase map Φ(x,y) of the test surface can be obtained, from which very accurate data about the surface profile may be produced using well known algorithms.
PSI provides vertical resolution in the order of 1/1000 of a wavelength or better; thus, it is well suited for characterizing smooth, well-reflecting surfaces and for identifying nano-size defects (in height) on a flat surface. At the same time, the PSI technique has a limited vertical range of application because of the so-called 2π ambiguity, which complicates the measurement when the surface features are higher than λ/2. Thus, in practice, conventional PSI techniques have been limited to the measurement of fairly smooth and continuous surfaces. When the objective of the measurement is the rapid detection of small defects in a large sample surface, conventional PSI is also limited by its intensive speed requirements for sample scanning and data acquisition.
Large-step, rough, or steep-surface measurements have been traditionally carried out with white-light (or broadband-light) vertical-scanning interferometry (VSI). However, with respect to the rapid defect-detection objective of the present invention, even greater speed limitations than PSI's apply to VSI. In addition, VSI's vertical resolution may sometime be insufficient to measure very small defects.
Thus, in order to speed up the measurement and produce meaningful results rapidly when a large sample surface is involved, the present invention adopts a single-frame raster-type approach for scanning the surface and a spatial-carrier method of analysis for processing the interferometric data collected during the scan. By introducing a tilt between the sample surface and the reference surface, each frame provides a set of fringes spaced apart according to the OPD produced by the tilt. If the sample surface were sufficiently flat to produce a reasonably uniform spacing between fringes, each frame could be analyzed using conventional PSI or Fourier Transform (FT) algorithms. For instance, when the tilt in the surface corresponds to fringes produced by a 90-degree phase change between adjacent detector pixels (the spatial phase step), a conventional 90-degree PSI algorithm applied to consecutive pixels may be used effectively to profile the surface and identify defects. Any discontinuity in the fringes will correspond to a discontinuity in the smooth surface profile (i.e., a defect) that can thus be identified and measured using the conventional temporal PSI algorithm. A similar analysis can be carried out with standard FT algorithms (see, for example, Mitsuo Takeda et al., “Fourier-transform method offringe-pattern analysis for computer-based topography and interferometry,” JOSA, Vol. 72, Issue 1, pp. 156-160).
Inasmuch as the tilt between a smooth surface and the reference surface can be controlled to produce the desired spatial phase step to match a particular algorithm selected for analysis, this single-frame approach is theoretically sound for finding defects rapidly and fairly accurately in large surface areas using conventional PSI or FT algorithms. However, all PSI algorithms are based on the information provided by a fixed number of consecutive irradiance data points acquired from the sample (at least three; typically five or eight). The most appropriate algorithm is selected according to the phase step expected between data points and a constant phase step is assumed.
As a result of the consecutive multiple data points used in a frame, PSI-type algorithms tend to smooth the phase information produced from the data acquired from the sample and the smoothing effect is greater when a larger number of points is used. This effect is even more pronounced with FT algorithms. Therefore, surface defects that involve very small areas, such as spots corresponding to one or two pixels of the detector, may be smoothed out and remain undetected if the wrong PSI algorithm is used with single-frame spatial-carrier data. Furthermore, even if the correct algorithm is selected for the particular defects expected to be found in a given sample, the effectiveness of the algorithm, which is founded on the presumption that fringes are produced by a constant phase step that is known a priori, is greatly undermined by slope variations normally present in the sample surface. Therefore, an unacceptable defect may go undetected or may be overlooked for further evaluation because the PSI algorithm is ineffective for the local changes in phase step size even though appropriately selected for the size of the defect.
In view of the foregoing, conventional single-frame spatial-carrier measurements, though rapid and potentially very advantageous for measuring large objects, are not effective for defect detection in large samples that are not perfectly flat. The present invention is directed at providing a solution to this problem.